This invention relates to method and apparatus to make a write once read many optical storage device appear to be rewritable.
Most presently used computer systems are designed to work with various types of data storage devices into which information can be written, read out, and rewritten many times. Examples of such devices include semiconductor memories and various magnetic storage media, such as magnetic disks and tape. While such storage devices have performed satisfactorily, the need for storing more and more information efficiently has driven the development of optical storage devices.
Currently available optical storage technology has provided a device with a characteristic commonly referred to as Write Once Read Many (WORM). Optical storage devices which are in the WORM family can only be written once because the medium is physically and permanently modified by the process of storing the information. The read process can then retrieve the information as many times as desired by detecting whether the medium has been modified or not. A problem created by WORM media is that most computer systems are not designed for a storage subsystem which restricts the number of times a "write" operation may be performed.
There are additional application problems caused by write-once media. Typical of these are computer operating systems which create many temporary files which are intermediate and transient to the operation of the system. Other problems concern the storage subsystem control structures which are constantly being modified and updated to reflect the current state of the subsystem. This kind of information is normally kept on the same subsystem which it controls. Thus, any time information is temporarily stored or stored information is modified, inefficient use of optical storage subsystems occurs because storage of temporary or intermediate files on WORM media wastes space. These problems stop optical storage devices from being used in normal ways. Virtually all current computer systems have not been designed to accommodate the restrictions WORM devices impose. The result is that integrating the WORM devices into current computer systems is extremely difficult and expensive. At present, only user-specific solutions have addressed these problems. As a consequence, the currently available optical storage options are not readily usable with a given computer system.
It is, therefore, an object of the present invention to provide a system to allow use of WORM media with the same ease as conventional storage devices.
It is another object of the present invention to provide a generally applicable solution to enable economical integration of WORM storage systems into existing computer systems.
It is a further object of the invention to provide a system which permits simplification of the tasks of integrating WORM media into a given computer system in a manner transparent to application software and at reduced cost and development time.